CODEWITHSUNDEEP
cpipeexec
Fu Hong
Fu Hong

Reputation: 9

Process gets stuck when I'm trying to recreate pipe/forking a new process

I'm trying to recreate pipe for a homemade shell and I can make the pipe work in this case:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

int main()
{
    int stat_loc;
    int pipefd[2];
    pipe(pipefd);

    pid_t pid = fork();
    if(pid != 0)
    {
        waitpid(pid, &stat_loc, WUNTRACED);
        close(pipefd[1]);
        dup2(pipefd[0], 0);
        close(pipefd[0]);
        execlp("cat", "cat", "-e", NULL);
    }
    else
    {
        close(pipefd[0]);
        dup2(pipefd[1], 1);
        close(pipefd[1]);
        execlp("ls", "ls", NULL);
    }
    //do something else
    printf("out to parent");
    return 0;
}

I would use the functional one above but I need to keep the parent process working.

But when I add one more fork, and the process gets stuck in

execlp("cat", "cat", "-e", NULL);

This is the full attempt that gets stuck:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

int main()
{
    int stat_loc;
    int pipefd[2];
    pipe(pipefd);

    pid_t pid1 = fork();
    if (pid1 == 0)
    {
        pid_t pid = fork();
        if(pid != 0)
        {
            waitpid(pid, &stat_loc, WUNTRACED);
            close(pipefd[1]);
            dup2(pipefd[0], 0);
            close(pipefd[0]);
            execlp("cat", "cat", "-e", NULL);
        }
        else
        {
            close(pipefd[0]);
            dup2(pipefd[1], 1);
            close(pipefd[1]);
            execlp("ls", "ls", NULL);
        }
    }
    waitpid(pid1, &stat_loc, WUNTRACED);
    //do something else
    printf("out to parent");
    return 0;
}

Thanks for the help!!

Upvotes: 1

Views: 128

Answers (1)

Barmar
Barmar

Reputation: 782529

You need to close all the pipe FDs in the parent process as well. Otherwise, cat will never read EOF from its input pipe.

Add:

close(pipefd[0]);
close(pipefd[1]);

before:

waitpid(pid1, &stat_loc, WUNTRACED);

Upvotes: 1

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